Something to think about:

While Griffith is human, Guts is an ordinary dude doing his best.

While Griffith is Femto, Guts is the Black Swordsman, traumatized, lashing out, driven by the rage inside of him.

While Griffith is NeoGriffith, Guts shifts away from rage and revenge towards personal growth.

Plus, each of Guts’ character shifts is a direct result of Griffith. Becoming Griffith’s soldier helps Guts come out of his shell, helps him grow as a person and accept the friendships of those around him, helps him be a responsible leader and dedicated comrade, and inspires him.

Guts becomes the Black Swordsman because of Femto’s actions, because of his desire for revenge and need to exorcise the rage and hurt inside him.

And Guts decides to take Casca to Elfhelm, gaining friends and chilling out along the way because NeoGriffith “abandoned” him.

I don’t have a real conclusion to draw but I’m throwing this out there anyway. Parallels, reflections, etc.